Our objective is to elucidate the effect of the heterosomes in the development of intermale aggression of inbred strains of mice in conjunction with: (1) different autosomal complements, and (b) early experience variables. The results of previous work suggested that appropriate models and methods for such investigations are available. Specifically, we have shown that the autosomal complement influences the Y-chromosome effect on intermale aggression and that either the autosomal complement and/or maternal environment has parallel influence on the size of the Y-chromosome. In the 1979-80 grant period, the following experiments are planned as an extension of these findings: (1) A study to determine whether or not traits such as testis size, sex ratio, male sexual behavior, and other types of aggressive behavior are influenced by effects of the DBA/1 Y-chromosome from characterized sublines in interaction with DBA/1 autosomes and/or maternal environment, and whether any of these are significantly intercorrelated. We have found subline differences in the physical characteristics of the Y-chromosome and in its liability for those in relation to differing autosomal backgrounds. (2) Continued development of congenic stocks differing in Y-chromosome from several DBA/1 sublines, DBA/2, C57BL/10, and C57BL/6 mice, which will be assessed for Y-chromosome size and intermale aggression at each generation of breeding. (3) A study of developmental changes in testosterone levels in the DBA/1 sublines and in the congenic stocks in association with the physical characteristics of the Y. (4) A study of Y-chromosome size and banding pattern in three DBA/1 sublines, DBA/2.